The present application pertains generally to disconnect switches, and more particularly to disconnect switches suitable for use in extra high voltage (EHV) and ultra high voltage (UHV) systems.
Disconnect switches are used to isolate a component of an electrical system from the power source, for example for the purpose of maintenance. Because such switches are normally operated in routine situations, rather than in emergencies, they do not require the extremely high ratings and great speed of operation required of a full interrupter. Nonetheless, because the continuous current requirements for these switches are generally high, the main contacts must be large. In this connection, it should be noted that just as 500 kV and 700 kV systems have been developed as an overlay for the more conventional 300-385 kV systems, so systems designed to operate at 1000--1000 kV are in development as a future overlay for 500-700 kV systems. In the new systems being developed, a disconnect switch may typically require a switch surge withstand capability of 2300 kV, an impulse withstand capability of 1120 kV, a power frequency withstand capability of 2000 kV and the ability to carry 760 kV relative to ground without visible corona (these figures are for a 1000 kV system).
It is difficult, and requires a great deal of energy, to move a contact of this size quickly, and it is typical for the moving contact of a disconnect switch to move at the relatively slow speed of 1 foot per second or less in being opened or closed. During the opening of a disconnect switch, as is well known, there is a risk of generating overvoltages if the switch restrikes across its contacts. The likelihood of such a restrike occurring is related to the rate at which the dielectric capability of the gap between the opening contacts increases. The slowness of the stroke of a disconnect switch means that this capability is increased very slowly, greatly increasing the likelihood of restrikes. Moreover, as the recovery voltage increases, reignitions and restrikes are increasingly likely to occur.
In the closing operation as well, overvoltages may be produced by prestrikes across the contacts. If the switch prestrikes, a line charging current flow is initiated, but as the current crosses its zero before the contacts of the switch have closed completely, the switch attempts to interrupt the current. Because the gap between the contacts is decreasing, although slowly, a voltage breakdown across the gap occurs. Again, because of the low speed of the moving contact and the correspondingly great time required for the closing stroke, the likelihood of overvoltages is increased.